A semiconductor unit that is provided with a semiconductor light-emitting device is commonly known. Such a semiconductor unit is produced by mounting a semiconductor light-emitting device on a submount 3 as shown in FIG. 7. FIG. 7 is a schematic cross-sectional view explaining a method of producing a conventional semiconductor unit. A method of producing a conventional semi-conductor unit is explained below by referring to FIG. 7.
As shown in FIG. 7, in a method of producing a conventional semiconductor unit 1, first, a submount 3 for mounting a semiconductor light-emitting device 2 on it is prepared. The submount comprises:                (a) a ceramic substrate 4;        (b) multi-layer that is formed on the substrate and that is composed of a layer comprising titanium (Ti) and a layer comprising platinum (Pt) (Ti/Pt multi-layer 5);        (c) a gold (Au) layer 6 that is formed on the multi-layer and that acts as an electrode layer;        (d) a solder-protecting barrier layer 107 that is formed on the Au layer and that comprises platinum (Pt); and        (e) a solder layer 8 that is formed on the barrier layer and that comprises gold (Au)-tin (Sn)-based solder.        
The Ti/Pt multi-layer, the Au layer, the solder-protecting barrier layer, and the solder layer may be formed by using a conventional layer-forming process, such as an evaporation method, a sputtering method, or a plating method, or a patterning process, such as a photolithographic method or a metal mask method.
After the submount is prepared as shown in FIG. 7, the solder of the submount is heated and melted to mount a laser diode as a semiconductor light-emitting device on a predetermined position on the solder (a die-bonding step is performed). Then, the back side of the submount is securely bonded to a heat sink (not shown in the drawing) with solder or the like to complete the production of a semiconductor unit provided with a semiconductor light-emitting device.
To reduce the damage of the semiconductor light-emitting device caused by the heating during the die-bonding step of the device, lead (Pb)-tin (Sn)-based solder or silver (Ag)-tin (Sn)-based solder, both of which have a lower melting point than the gold-tin-based solder, is sometimes used for the solder layer. When the silver-tin-based solder is used, a lead-free production can be achieved at the same time.
In recent years, the increase in the writing rate of the CD unit and the DVD unit and the increase in the output of the laser-processing machine have been increasing the output of the semiconductor light-emitting device. The semiconductor unit to be used in those units and machine is required to have high reliability in use. To meet this requirement, it is necessary to increase the bonding strength between the semiconductor light-emitting device and the submount.